1. Field of the Invention
This application relates to communication cables. More particularly, this application relates to loose-tube type fiber optic cables.
2. Description of the Related Art
In the area of fiber optic cables, there are many different designs, each of which has some purpose both in fiber count, mechanical properties, environmental resistance properties, fire resistance/smoke, etc. . . . . Among the various designs, mid count-designs (eg. more than 12—less than 100 fibers) typically contain the fibers in a loose tube style arrangement. “Loose tube” is a commonly understood term designating a fiber cable design that has a jacket, at least one buffer tube inside the jacket with at least one (usually more) UV coated optical fiber(s) loosely contained inside each buffer tube.
More particularly, the “loose” term in “loose tube” refers to the fibers being loose within buffer tube thus allowing the fibers to reside within a relatively free space. Within this free space the fibers have the ability to bend/move (such as into a sinusoidal shape) along the length of the cable, accumulating as the cable (jacket and tubes) contracts over cold temperature extremes. By allowing for this “loose” room with the buffer tubes, the fibers are able to avoid the stresses imparted by the cold temperatures on the tubes and jacket and thus likewise avoid undue attenuation.
FIGS. 1 and 2 show typical prior art loose tube fiber optic cables for forty eight (48) fibers (FIG. 1) and seventy two (72) fibers (FIG. 2). In these figures, a jacket is provided with either four or six buffer tubes each having twelve (12) loose fibers therein. Each of the tubes are arranged around a central strength member, typically a GRP (Glass Reinforced Polymer) to provide various mechanical advantages to the cable including longitudinal strength, and cold temperature contraction resistance, cable resistance, etc. . . . . In these present examples in FIGS. 1 and 2, a few (e.g. three) aramid strength strands are added to the interior of the tube(s) to aid with the process of adding connectors to the fibers at the ends of tubes (i.e. once they are removed from the cable for connectorization).
Although this design is adequate for many purposes it has certain drawbacks particularly with cold temperature resistance for designs having four (4) tubes because the center space is small relative to the cold temperature contraction forces of the tubes and jacket areas. As noted above, a normal design feature for fiber optic cables is a cold temperature resistance rating, meaning that the cable does not overly attenuate at some particular cold temperature. For example, in many designs for mid-count fiber optic cables, there is a maximum allowable attenuation of 0.3 db change under low temperature conditions (typically either 0 C, −20 C or −40 C).
The reason this is important is that the polymers typically used for a jacket and the buffer tubes, such a PE (polyethylene), PVC/FRPVC (polyvinylchloride/flame retardant polyvinylchloride), FEP (Fluorinated Ethylene Polymer), PVDF (Polyvinylidene Fluoride) etc. . . . , shrink a good amount, eg. between 0.2% to 1.5% shrinkage through the transition from room temp (23° C.) to cold temperatures (to −40° C.). Likewise, the UV coated glass fibers, loosely contained in the buffer tubes, also contracts in the cold, but to a much lesser extent, e.g. approximately 0.08%-1%. Moreover, the Aramid in the outer layer actually undergoes a moderate expansion in cold temperatures. The GRP, a composite of glass and plastic, only contracts a slight amount, e.g. approximately 0.04%.
The contraction in cold temperatures of the tubes and jacket to a much greater extent than the fibers in the tubes (and the GRPs), results in the fibers gaining excess length relative to the tubes they are contained within, resulting in the fibers assuming an exacerbated sinusoidal shape after the interior spiral helix space within the loose tube stranding is consumed. Thus, as the temperature continues to go down, the attenuation in the fibers goes up resulting in the cable eventually failing. This situation can actually be further exacerbated in the situation where aramid yarns are in the tubes since that further limits the available free space in the tubes for the fibers to adjust.